In his Hitchhiker’s Guide to the Galaxy books, the late, great British satirist Douglas Adams wrote that dolphins are the second most intelligent creatures on earth — before humans and after mice, which spend their time running complex lab experiments on scientists. The mice might not quite live up to their No. 1 billing, but the more we learn about the cognitive abilities of dolphins, the more they indeed seem to have the No. 2 spot locked up. Not only do dolphins have impressive memories for tasks, the ability to use tools and elaborate social structures, but they also have their own names, distinctive identifying whistles that they develop themselves. Now a study in Proceedings of the Royal Society B reports that dolphins can recognize the whistles of others they shared a tank with as long as 20 years ago, the most enduring social memories ever observed outside of humans.

The study, undertaken by Jason Bruck, then a graduate student at the University of Chicago, used 43 dolphins ranging from 4 months to 47 years old that are cycled among various institutions as part of a captive breeding program. Some of them had lived together for only three months; others had shared a tank for more than 18 years before being separated. Bruck first obtained recordings of each dolphin’s name whistle. Then he set up an underwater speaker in the dolphins’ tanks that played whistles from strangers and from former tankmates. The difference in the dolphins’ reactions was unmistakable.

“When they hear a dolphin they know, they often quickly approach the speaker playing the recording,” Bruck said in a prepared statement. “At times they will hover around, whistle at it, try to get it to whistle back.” They paid far less attention to a stranger’s whistle-name.

Moreover, it didn’t matter how long the animals had been separated — a dolphin could recognize the call of a companion it had last seen decades ago just as easily as one it last saw six months ago. Nor did it matter how short or long the animals had been housed together; they responded with the same recognition to a long-term friend or a more fleeting acquaintance. In the most impressive case, a dolphin named Bailey recognized the whistle of Allie, her tankmate 20 years and six months ago. “This shows us an animal operating cognitively at a level that’s very consistent with human social memory,” Bruck said.

Wild dolphins have a life expectancy that ranges from 20 to 50 years (though such superannuated adults are rare), and they live in ever shifting pods, with individuals constantly splitting off and reuniting with the group. Bruck suggests that animals with such a social structure may benefit from a long memory for one another, perhaps supporting a connection between complex social behavior and the evolution of memory. But it may also be that a prodigious memory for names is just part of the larger, fascinating package of dolphin intelligence, included for no particular evolutionary reason — another element of the hidden depths of a mammalian cousin that we continue to explore.